Polling data is goofy anyway, there is only so much one can really learn from them. Still, this undercount seems to me worthy of note.

Say you want to reach a representative sample of the U.S. electorate for a presidential poll. The Obama-McCain race is relatively close these days, with the Democrat’s lead hovering around 5 to 6 points in most surveys. Someone tells you that he’s selected a sample that’s predominantly under 40 years of age (oops, that one favors Obama); disproportionately renters rather than homeowners (Obama-leaning again); full of college students (sounds like a Starbucks Obama thing to me) — and, for good measure, includes a higher proportion of blacks and Hispanics than the national population does.

At this point you throw up your hands and exclaim: “Why are we concentrating on such a pro-Obama universe? He could be leading by 20 points or more among those people!”

He could. He probably is. But in actuality, the sample I’ve described is either not being included at all in many national polls or is being undercounted. Why? Because I’m talking about the growing number of American cellphone users who have no other type of phone or who choose to go wireless for the vast majority of their interactive needs. And this election cycle — for the first, and perhaps only, time — this group has the chance to render presidential polls “wrong from the start”: potentially disguising at least 2 to 3 percentage points of Obama support and maybe more.

Heretofore my industry has dismissed the cellphone-only population with a troika of “yes, buts.” Yes, they’re undercounted, but 1) they don’t vote anyway; 2) their numbers are still small; and 3) we can find acceptable substitutes in the land-line population.

And to be honest, there is a fourth, still more powerful rationale that remains unstated: “Yes, they’re undercounted, but it’s too damn difficult and expensive to reach them.”